In recent years, the rapidly growing popularity of the Internet has caused a strong need for an increased information transmission capacity of optical fiber communication networks, and as a means to this end, the development of wavelength-division multiplexing (WDM) has advanced rapidly. In optical communication by wavelength-division multiplexing, light beams with slight wavelength differences transmit information separately, so that it is necessary to use such optical functional elements as spectral separators, filters and isolators having high wavelength selectivity. What is required of these functional elements is suitability for mass production, compactness, high integration and stability.
Spectrometers are used to separate and detect optical signals in which a plurality of wavelengths are multiplexed intentionally, as in wavelength-division multiplexing. Moreover, spectrometers are used for spectral analysis of measured light, as in spectrometry. Spectrometers are also used in optical disk systems utilizing light sources with a plurality of wavelengths. Such spectrometers require a spectral separation element such as a prism, wavelength filter or diffraction grating or the like.
In particular diffraction gratings are typical spectral separation elements. Diffraction gratings are fabricated for example by forming a periodic blazed microscopic relief structure in a surface of a quartz or silicon substrate, for example. The diffraction light rays generated by this periodic relief structure interfere with each other, so that light of certain specific wavelengths is emitted in specific directions. Since diffraction gratings have such characteristics, they are used as spectral separation elements. Such spectral separation elements have been disclosed in JP H10-300976A, for example.
It is generally known that the wavelength resolving power of diffraction gratings is proportional to the product of the order of the diffraction light and the groove number. What is effective in actual spectral separation elements is the periodicity of the diffraction grating in the range through which the light beam passes. That is to say, in order to improve the resolving power of a diffraction grating, it is necessary to make the diameter of the light beam larger. In order to make the diameter of the light beam larger, the components that are necessary for the optical system accordingly need to be made larger as well.
However, making the optical components, such as the diffraction grating or lenses etc., larger leads to higher costs. Also, as the light beam becomes wide and the lens diameter becomes large, aberrations tend to increase as well, so that means for correcting aberrations need to be provided. Therefore, there is also the problem that spectrometers tend to become large.
Blazed gratings with a sawtooth-shaped cross-sectional shape often are used as diffraction gratings with the purpose of spectral separation. There are reflective and transmissive diffraction gratings having a blazed grating. FIG. 13A is a cross-sectional view showing the configuration of a transmissive diffraction grating 103a having a blazed grating, and FIG. 13B is a cross-sectional view showing the configuration of a reflective diffraction grating 103b having a blazed grating. In the transmissive diffraction grating 103a shown in FIG. 13A, when light 107a of a plurality of wavelengths is incident on the side opposite the side on which the grooves 104a are formed, then a plurality of diffraction light beams 108a and 109a are emitted, which are separated spectrally due to the different directions in which they are emitted from the side on which the grooves 104a are formed.
In the reflective diffraction grating 103b shown in FIG. 13B, when light 107b of a plurality of wavelengths is incident on the side on which the grooves 104b are formed, then it is reflected and a plurality of diffraction light beams 108b and 109b are emitted, which are separated spectrally due to the different directions in which they are reflected from the side on which the grooves 104b are formed. With the reflective diffraction grating 103b, a higher diffraction efficiency can be attained than with the transmissive diffraction grating 103a, so that commonly, the reflective diffraction grating 103b is used. However, with the reflective diffraction grating 103b, it is necessary to process its surface into a reflective surface.
In both the transmissive diffraction grating 103a and the reflective diffraction grating 103b, when the grating period approaches the wavelength of the light, differences in efficiency occur that are dependent upon the polarization direction (TE polarized light or TM polarized light). For this reason, precise design of the blazed shape and a sophisticated machining technology are necessary in order to attain a high diffraction efficiency.